Heat sensitization of aluminum 5xxx aluminum-magnesium alloys is associated with the formation of a magnesium-rich (compared to the solid solution) Mg2Al3 phase on the grain boundaries when the alloy is exposed to elevated temperatures. This magnesium-rich phase, known as beta phase Mg2Al3, or often simply as “beta phase,” on the grain boundaries is anodic with respect to the surrounding aluminum-magnesium solid solution, thus presence of beta phase on the grain boundaries increases the potential for intergranular corrosion, stress-corrosion cracking, and intergranular corrosion-fatigue, leading to degradation of ship structure mechanical reliability.
Conventionally, the degree of sensitization (DOS) is characterized with the ASTM G67 Nitric Acid Mass Loss Test set forth in ASTM G67-13, “Standard Test Method for Determining the Susceptibility to Intergranular Corrosion of 5XXX Series Aluminum Alloys by Mass Loss After Exposure to Nitric Acid (NAMLT Test),” (2013) available from ASTM International, Inc. In this test, a specimen of the material in question is immersed in temperature-controlled concentrated nitric acid for a period of time, and the amount of mass lost from the specimen after the test versus before the test is measured. Essentially, the test is contrived to allow the acid to severely etch the grain boundaries, and the result is that grains fall out, accounting for the mass loss. Obviously this approach is destructive and generates undesirable hazardous waste, and so cannot be used in-situ on a ship.
The total amount of beta phase present in the worst case of sensitization is very small, while the effects on corrosion of grain boundaries is very high. A number of approaches for assessing the DOS based on measures of the amount of beta phase in the bulk material have been tried or speculated about, including use of microwave cavity resonance perturbation, electrical conductivity, hardness, ultrasonic attenuation, x-ray composition analysis, and so on. See, e.g., C. Chukunonye, “Sensitization Characterization of 5083 and 5456 Aluminum Alloys using Ultrasound,” Dissertation, University of Louisiana at Lafayette (2015) (“ultrasonic method”); M. Shedd, G. Bunget, F. Friedersdorf, and N. Brown, “Embedded Long Service Life Monitoring System for Aluminum Alloy Sensitization,” ASNE MegaRust Conference, (2013) (“eddy current method”); and B. A. Shaw, “Fieldable Probe for Quantitative Assessment of Degree of Sensitization in Marine Aluminum Alloys,” 2009 Navy SBIR Topic N09-T022, Award 90313 (2009) (“x-ray diffraction method”).
Certain mechanical properties of the alloys, such as hardness, also correlate with the DOS. See I. N. A. Oguocha, 0. J. Adigun, and S. Yannacopoulos, “Effect of sensitization heat treatment on properties of Al—Mg alloy AA5083-H116,” J. Matter. Sci. (2008) 43:4208-4214. These correlations also could form the basis of a DOS measurement approach. Hardness in particular is a very easy measurement to make. Researchers at the Naval Research Laboratory have done exploratory research on using hardness measurements for estimating DOS. The hardness, for example Rockwell Hardness or Vicker's Hardness, decreases rapidly with degree of sensitization. However this approach requires a known unsensitized reference sample of the material being tested, which is not always available.
In addition, beta phase can exist intragranularly in addition to being on the grain boundaries, though the intragranular beta phase does not affect the intergranular corrosion significantly. Thus techniques that seek to assess DOS based on measures of the amount of beta phase in the bulk tend to be either insufficiently sensitive, or yield overestimates of the effect of grain boundary beta.
An alternative to the ASTM G67 DOS test is the portable electrochemical system known as the ElectraWatch DoS Probe developed under funding from the Department of the Navy. See Electrawatch, Inc., Degree of Sensitization (DoS) Probe; see also W. J. Golumbfskie, K. T. Tran, J. M. Noland, R. Park, D. J. Stiles, G. Grogan, and C. Wong, “Survey of Detection, Mitigation, and Repair Technologies to Address Problems Caused by Sensitization of Al—Mg Alloys on Navy Ships,” CORROSION, Vol. 72, No. 2, pp. 314-328. The ElectraWatch DoS Probe works by measuring the electrochemical currents and voltages as the beta phase undergoes a reaction when a reagent is applied to the surface. While this works well in the laboratory, it is a specialized instrument that must have access to a large flat area of surface to seal against, to contain the electrochemical reaction volume. Further, it can be temperature sensitive and requires specially trained personnel to operate.
A viable DOS characterization tool must be specific to grain boundary beta phase. Microstructure analysis studies by NRL, see R. Goswami and R. L Holtz, “Transmission Electron Microscopy Investigations of Grain Boundary Beta Phase Precipitation in Al 5083 Aged at 373K,” Metallurgical and Materials Transactions A, 44A, pp 1279-1289 (2013), and others have shown that high values of DOS as measured by the mass loss test are directly associated with the degree of coverage of the grain boundaries by beta phase. When the beta phase is present primarily as isolated, precipitates, the G67 mass loss is very low. When the beta phase is a continuous or nearly continuous layer, the mass loss is very high. While the exact role the continuity or topology of the beta coverage plays in G67 mass loss is not entirely known, it is sufficiently clear that measuring beta coverage of the grain boundaries provides a metallographic option for estimating DOS, if calibrated against G67. Such an approach would be non-destructive to the bulk of the material, thus could be used in-situ in some circumstances, and eliminates the generation of large amounts of acid waste.
Metallographic etching can be used to dissolve beta phase in aluminum-magnesium alloys, and if the beta phase is concentrated along grain boundaries, the etching patterns are easily seen with optical metallography. Qualitative assessment of sensitization via etching and metallography has been commonly done since the sensitization phenomenon was first identified. This is suggested in ASTM B928; see also S. Jain, J. L. Hudson, and J. R. Scully, “Effects of constituent particles and sensitization on surface spreading of intergranular corrosion on a sensitized AA5083 alloy,” Electrochimica Acta 108 (2013) 253-264. However, prior to the current invention disclosure, no quantitative metallographic technique has been developed for this purpose.